This invention relates to devices known as liquid level detectors, interface probes, and pressure transducers which are used to gauge the depth of groundwater and/or any light non-aqueous phase liquids (LNAPL) or dense non-aqueous phase liquids (DNAPL) i.e. petroleum or solvents respectively, Traditionally, in accordance with the prior art, monitoring wells are gauged using invasive detection devices with a sensor probe attached to a graduated tape that is wound on a reel. The sensor probe is lowered into the monitoring well casing until the sensor probe comes in contact with the subsurface media i.e. groundwater, in the well. To detect liquids, these devices use an infra-red emitter and detector. When the probe enters a liquid the infra-red beam is refracted away from the detector, which activates an audible alarm and light. If the liquid is non-conductive (NAPL) the alarm and light are steady. If the liquid is water, a conductive liquid, the water completes a conductivity circuit. This overrides the infrared circuit, and the tone and light are intermittent. The total depth of the well is determined by lowering the sensor probe to the bottom of the well. The measurement is read from the graduated tape and the depth of each media is manually recorded. Errors can be introduced through transcription and by misreading the graduated tape.
If a non-aqueous NAPL such as petroleum is present, the sensor probe and measurement tape can become smeared with petroleum, which in some cases can be difficult to completely remove from the sensor probe and tape. The decontamination process must be thorough to avoid leaving behind any petroleum residues. In accordance with state and federal environmental regulations, these devices must be thoroughly decontaminated between use in each monitoring well to prevent cross-contamination from well to well in subsurface groundwater. This is an important step in the prior process which is time consuming and consequently costly. In addition, when the probe is removed the field personnel gauging the well, run the risk of being exposed to contaminants in the well.
The present invention avoids prior art tape-reading errors and transcription errors by displaying the gauged values on a hand-held digital display unit where they can be stored electronically and later transferred directly to a computer. There is no decontamination procedure necessary, since the sensor tape is part of the well and there is no contact with contaminated groundwater which can be transferred from one well to the next or from the well to field personnel.
Regarding groundwater sampling and well volumes, in order to determine groundwater quality it is necessary to collect samples of groundwater and submit them for laboratory analysis. Prior to collecting a groundwater sample a specified volume of groundwater (typically 3 well volumes) must be withdrawn from the well in order to obtain a sample representative of the aquifer. The volume of groundwater within the monitoring well casing is calculated using the measured depth to water and total depth of the well. In addition it is necessary to determine the volumes of any NAPL that may be present in a well to determine the extent of release of such liquids. The invention disclosed herein, calculates and displays the thickness and volume of water and any LNAPL or DNAPL on the hand held digital display unit.
Aquifer tests such as pumping tests and slug tests are used to determine site-specific aquifer parameters such as hydraulic conductivity, transmissivity and storage coefficients. These tests are performed while groundwater levels in monitoring wells are continuously monitored and recorded. This is typically accomplished through the use of a pressure transducer probe installed in a monitoring well, penetrating the groundwater and attached via chords to a data logger at the surface. These data loggers collect and store the groundwater level data for subsequent determination of aquifer parameters. The pressure transducers must also be decontaminated between each use to prevent cross-contamination.
The invention disclosed herein can also monitor and store groundwater level data during these aquifer tests for subsequent determination of aquifer parameters.
Regarding well locating, it is sometimes difficult to determine if the well being gauged or sampled is the well of interest. This is particularly a problem at sites with numerous wells located relatively close together and installed at similar depths. Groundwater data collected is invaluable in determining site conditions accurately and gauging and sampling the correct well is paramount.
In the aforesaid prior art apparatus, an elongated resistive sensor tape was employed to detect the level of a liquid using the hydrostatic pressure of the liquid in which it is immersed. The hydrostatic sensor consists of a conductive base strip that is partially insulated from a resistive wire that is wound around the base strip to form a helix. The sensor tape is encapsulated in an outer protective envelope to insulate the sensor from the liquid in which it is immersed. When the sensor is immersed in a liquid the hydrostatic pressure of the liquid compresses the envelope. This causes the wire to contact the base strip, which results in a change in resistance of the wire. The resistance of the wound wire corresponds to the distance from the top of the sensor tape to the liquid surface. See U.S. Pat. No. 4,816,799 to Ehrenfried et al., assigned to Metritape Inc. of Littleton. Mass.
A disadvantage of this prior art sensor is that it only can detect the level of a single liquid, e.g. water or petroleum. The sensor can not distinguish between the two in the same well, tank or vessel. In addition, the resolution of the sensor is limited by the minimum spacing between windings of the resistive wire (two hundredths of a foot).
The invention disclosed herein, utilizes an elongated resistive sensor tape that incorporates both a hydrostatic and conductive resistive circuit capable of distinguishing between a conductive liquid (water) and a non-conductive liquid (petroleum) at a resolution of one hundredth of a foot and greater. The resolution is limited only by the smallest spacing possible between contacts of individual thin film transistor circuits in the neighborhood of micrometers.